hmm....I think I understand it. It's all about force, and in this case, the ONLY source of the force is the wind on those propellor blades.If they are large enough, there is enough force to play with, and applying that to drive wheels seems to be part of the concept here. Pretty fun stuff.

BUT, and this is a big but...this original concept starts out stating that the purpose is to SAIL faster than the wind. SAIL.

Sails do not rotate. Nor do they have propellors, nor rotation shafts, nor bearings. Is a gyrocopter a sailplane?

Nope.

He might be using the wind to move faster than the wind, and using the rotation of the prop to re-align the propellor/ sails actual path across the wind(the "sail" part of the prop is, in fact, tacking across the wind, correct?) but he isn't "sailing" per se.

Truly amazing stuff, while this has been used with land yachts i see no reason why it could not be used on water with a prop or modified mirage drive.

I think there could be some improvements using the thrust of a mirage drive mechanism tied to the power from the sail but I doubt a TI could go directly downwind faster than the wind due to too much drag and coupling inefficiencies in water compared to a land sailing craft. But it is fun to explore some possibilities.

Imagine a vertical windmill in the mast slot of the TI with two drive belts connecting a wheel on each side mounted between the hull and amas and those wheels driving submerged flexible flippers on each side creating forward thrust like sculling oars. Since vertical windmills spin regardless of wind direction you could sail in any direction. But I doubt it would go faster than a normal TI due to drag from half the surface of the windmill (half of area of vertical windmill going into the wind) so nothing really gained.

If used a large horizontal propeller in the air on a hydrofoil sailboat then perhaps the drag would be low enough to go directly downwind perhaps faster than the wind provided you used some external power (only initially) to get it up on the foils. The trick would be to transmit a submerged propeller's energy to the air propeller above such that it rotated not as a windmill absorbing energy but in the opposite direction providing forward lift. But I would expect it wouldn't work because the losses coupling the submerged prop to the air prop would be much greater than the wheels coupling the air prop on the land vehicle though all other physics principles would apply.

It also probably wouldn't work for the iceboat version of a TI since difficult to transmit energy from the relative motion of the ice to the air prop - in that case due to too low friction with the ice.

It is possible to make a TI version that could go directly into the wind using a horizontal air prop but it would not be practical due to the danger of the spinning prop and it would probably go much slower than a normal stationary sail TI so would lose any upwind race.

However, there may be some improvements in the speed of the conventional sail TI in low wind conditions using the same principle of the thrust of the mirage type flippers if coupled to the energy of the conventional sail such that its angle oscillates slightly about its average optimum through a feedback loop from flipper propulsion.

You can test this flipper propulsion phenomena yourself now when sailing upwind in very light wind conditions by oscillating the rudder very slightly on an upwind tack and noticing the increased speed. The entire submerged TI boat surface seems to be acting like a sculling oar. Now just replace that tiny energy of your hand moving the rudder with the energy you use to hold the main sheet and imagine a flexible submerged flipper to amplify the thrust more efficiently and the motion of your hand replaced with an oscillation from a feedback loop powered through the force of the main sheet coupled through a pulley mechanism tied to a submerged flipper in one of the TI mirage drive wells.

I think I am still struggling a little with what force is acting on the prop when it is at zero wind speed. What accelerates it past that? Where is additional energy being added? The lift that sucks the propellor around is from air flowing over it.

So, is the prop direct drive to the wheels, and the inertia of the moving cart is also driving the prop around?

After giving this more thought I have some issues with the logic. At a dead stop with the wind coming from behind the prop will turn in a particular direction. As you reach a land speed equal to wind speed the wind will stop (he mentions that the tell tales go slack) and at this point only the momentum of the vehicle will keep the prop turning (as mentioned in the post above). As you accelerate (as described in the article) to a speed faster than the wind the wind will start to come from in front of the vehicle and the prop will need to stop and start spinning in the opposite direction or have a variable pitch prop that is reversed at that point. If that worked then you could also take the same vehicle in zero wind, give it a push to get started and it would just keep accelerating.

I mean, what would it take to get the cart up to that speed with no wind connection at all? Something like a 3 HP Briggs and Stratton?

If that cart had wings, could he then fly across the Atlantic west to east with no source of energy other than a tailwind on his pusher prop? I mean, if this is all true, the friction at the earth interfaceis the only thing holding it back, right?

IF this all works as described, can someone explain to me why you don't get holes drilled through you if you stand downwind of a maple grove in a stiff breeze when the seeds are falling?

and the prop will need to stop and start spinning in the opposite direction or have a variable pitch prop that is reversed at that point. If that worked then you could also take the same vehicle in zero wind, give it a push to get started and it would just keep accelerating.

I am thinking that the test results are not possible.

I was thinking exactly the same thing Chet3 but then realised, after looking at the footage here http://www.fasterthanthewind.org/that on initial startup, the prop rotates in the opposite direction than we both first though. Have a look. The wind from behind doesn't turn the prop, the wheels do. And what turns the wheels? The wind resistance on the prop. Even though it's turning in the wrong direction, it is still offering enough resistance to push the wheels forward. Once it gets going, it gets easier and easier to get up to wind speed.

After that..........dunno...........I need more time to think, but I'm getting there.

I had not seen the video before or the site where it is located. It sure looks legit in the video and you are correct about how the prop spins, very clear in the video and different than what I was thinking.

The following explanation seems to help me think through how a wind powered vehicle can go directly down wind faster than the wind:

A propeller can be used either to absorb wind energy as a windmill and drive a load or to act as a load by providing thrust and be fed by an energy source such as in a plane. As mentioned earlier, to go faster than the wind directly downwind, the prop spins in the opposite direction as it would if used as a windmill so is acting as a load providing thrust and it is energized by the motion of the vehicle relative to the ground since the wheels are geared to the prop for that rotation direction. (Perhaps if geared to rotate in the opposite direction then it could be used to go directly into the wind and possibly faster than the wind.)

But in the down wind example, as the wind blows the vehicle directly downwind, the wheels start to turn and so the prop turns and starts to provide some weak thrust. That thrust provides a force in addition to the push from the tail wind and so it accelerates the vehicle which causes the wheels to spin faster and provide more speed to the prop and therefore more thrust and so on accelerating the vehicle downwind faster and faster right through the zero apparent wind where it now changes direction and becomes an apparent head wind.

And here is the counter intuitive part - that apparent head wind now provides a force that tries to slow the vehicle down yet that same head wind also assists the prop since now reinforcing its rotation as a thrust force pushing the vehicle downwind still faster and so it continues to accelerate until the drag force of the apparent head wind is exactly matched by the thrust of the prop - and that speed was close to 3 times the speed of the wind. Since it is being powered in real time by the wind (no stored energy), it follows that if that tail wind suddenly shut off, the apparent head wind should increase instantly providing a sudden jump in the drag force which had just before been exactly balanced by the prop's thrust but now exceeds it so the vehicle slows down so less ground speed and less prop thrust and so on coming to a stop.

I believe that there is or will be soon a demo of going directly into the wind faster than the wind in a land "sailing" vehicle but geared for the opposite direction of rotation.

Interesting to imagine how to apply these ideas to a watercraft using a normal sail which is connected to a mirage type of fin propulsion to increase the speed on an upwind tack.